The invention relates to the laser trimming of films deposited on substrates. In particular, the invention is useful in the laser trimming of so called thick film resistors. Typically a high alumina substrate is provided with a pattern of metal resistor terminals. Then a layer of usually proprietary composition is deposited or screened onto the substrate so that a continuous film extends between the metal terminal ends of resistor elements. The deposited material is then fired at elevated temperature to cause the screened composition to set into a state that is stable and yields suitable resistor characteristics. Since such a screening process will produce only approximately correct resistor values it is common practice to trim the deposited films after firing to achieve a precise resistor value. Normally the resistor is manufactured to a value that is reliably below the target value. Then each one is trimmed to the desired value and precision.
Laser trimming has proven to be exceptionally well suited to such trimming. Normally the laser is focused to its smallest spot size on the ceramic substrate surface. Then the power is adjusted until a clean kerf with a suitable crystalline edge is produced. Trimming is then accomplished with occasional, readjustment as desired. Obviously, this procedure is highly subjective and even with highly trained operators, is not reproduceable. If too much laser power is employed, there will be excessive destruction of the substrate and possibly substrate burning. Too little power results in a ragged kerf which makes the trimming unpredictable.
In terms of laser focus the shape of the typical laser focal saddle makes it difficult to observe. If the focus is incorrect the actual hot spot may be elliptical so that cuts along one ellipse axis will be wider than perpendicular cuts. With this condition the laser trim rate will be different for various cut directions. When the focus is proper, the hot spot will be round and the material removal rate will be the same in all cut directions. It turns out that the actual focus should be slightly below the substrate surface. Typically it has been found that the best focus point is about one-half micron below the ceramic surface. As a matter of experience it has been found that focusing the laser is a trial and error process highly dependent upon the skill of the operator.